1. Field of the Invention
The present invention relates generally to a temperature compensation circuits, and more particularly to circuits that compensate the temperature versus frequency characteristic of variable frequency oscillators, such as Surface Acoustic Wave resonators.
2. Description of the Related Art
An oscillator's output frequency will drift from a target, or center, frequency with variations in temperature. In precision frequency devices, frequency drift due to temperature variation is of primary concern since an oscillator's ambient temperature will fluctuate under normal use. Therefore, it is customary to provide an oscillator circuit with temperature compensation circuitry to attempt to stabilize the oscillator's output frequency over a predetermined operating temperature range.
Voltage (VCO) or current (CCO) controlled variable frequency oscillators are frequently used in precision clock generators for digital systems. These oscillators typically have a function control mechanism to adjust their frequency in accordance with system frequency variations. In such systems, it is important that the functional control signal additionally include sufficient signal control range to compensate for the frequency variations of oscillation due to circuit temperature variations. Thus, the temperature control range is in addition to the required range of the functional control mechanism used to compensate for normal system frequency variations. This temperature compensation requirement reduces the functional control pull range of the oscillator which imposes more stringent requirements on its functional behavior.
The basic ideal of temperature compensation for oscillators is relatively straight forward. With reference to FIG. 1, a typical temperature compensated oscillator consists of temperature compensation signal generator 1001 coupled to a control input of a variable oscillator 1002. Ideally, temperature compensation signal generator 1001 includes a temperature sensor for monitoring temperature changes, and generates a signal selected to increase or decrease the frequency of variable oscillator 1002 in such a way so as to counteract the natural frequency drift of variable oscillator, 1002 due to temperature variation. That is, if a temperature change would naturally cause the operating frequency of uncompensated variable oscillator 1002 to drift upward, i.e. increase, temperature compensation signal 1001 would preferably produce a signal instructing variable oscillator 1002 to lower its operating frequency by an amount sufficient to counteract the oscillator's temperature induced, natural upward drift. Similarly, if the frequency of variable oscillator 1002 were to naturally decrease due to a change in temperature, temperature compensation signal 1002 should instruct variable oscillator 1002 to increase its operating frequency to counteract this downward drift.
In practice, however, it is difficult to construct a temperature compensation signal generator that produces an output compensation signal that varies with temperature in manner sufficiently inverse to that of variable oscillator 1002 over a wide temperature range. This is especially true of high precision oscillators. Prior art temperature, compensation signal generators typically use a diode as a temperature sensor since a diode's characteristics are uniformly dependent on temperature, as shown in U.S. Pat. No. 5,097,228. However, an oscillator's natural frequency drift due to temperature might not be uniform with temperature. That it, within a first temperature range, the oscillator's frequency may naturally increase with increasing temperature, but within a different temperature range, the same oscillator's frequency may naturally decrease with increasing temperature. Thus direct use of a temperature sensor uniformly dependent on temperature to compensate an oscillator's temperature drift may not be suitable for all oscillators over extended temperature ranges.
To address this non-uniformity in the frequency drift of some oscillators, another approach attempts to compensate for temperature variations by first observing an oscillator's natural (i.e. uncompensated) temperature drift over a specified temperature range of interest. The observed frequency variations are digitally recorded in a memory and compensation signal values corresponding to each observed frequency variation are likewise recorded in the memory. In operation, a temperature sensor diode is used to monitor temperature variations, and the output from the sensor diode is applied to an analog-to-digital converter to obtain a digital representation of the temperature variation. The digital representation of the temperature variation is used to lookup its corresponding compensation signal value in the memory. The thus acquired compensation signal value is applied to a digital-to-analog converter to create an equivalent analog compensation signal that is applied to the oscillator. A similar type of temperature compensation control is shown, for example, in U.S. Pat. No. 5,604,468. However, this approach to temperature compensation requires much additional circuitry, much initial testing to setup a look-up table in the memory, and added complexity.
U.S. Pat. No. 4,492,933 to Grieco shows a temperature compensation circuit that avoids the use of A/D and D/A converters and does not require digital memory for compensating an oscillator having a frequency response that is parabolic (i.e. non-uniform) with respect to temperature. However, Grieco's approach still requires discrete jumps in the compensation signal, as well as complicated circuitry, such as differential amplifiers, integrators, one-shot circuits, and sample-and-hold circuitry, all of which place operating frequency limitations on the compensation circuitry as well as complicating its construction.
With reference with FIG. 2, Grieco explains that the frequency-to-temperature characteristic curve of a surface acoustic wave (SAW) device is a concave down parabolic curve 1011, and that a temperature compensation curve 1013 should therefore ideally be a concave up parabolic curve 1013 since its shape ought to be the inverse of the temperature characteristic curve 1001. To generate a concave up curve, Grieco divides the temperature range of FIG. 2 into a “cold end” (the rising part of curve 1001 up to its apex) and a “hot end” (the falling part of curve 1001 down from its apex).
Grieco first explains how to create a half-parabolic curve having a generally concave up shape to compensate the “cold end” of curve 1011. To achieve this, as shown in FIG. 3, Grieco provides a temperature sensor circuit 1014 whose output voltage 1016, as it would be understood in the art, drops as temperature increase, as indicate in FIG. 4.
With reference with FIG. 5, Grieco also provides a series of linear timing ramps 1015 to define sampling periods SP1 to SPn. Linear timing ramps 1015 are integrated to generate a series of half parabolic curves 1017 whose periods are defined by the sampling periods SP1 to SPn. Within each sampling period, the voltage of a half parabolic curve 1017 is sampled when the magnitude of the linearly increasing ramp 1015 rises above output signal 1016, which is a measure of temperature as determined by temperature sensor circuit 1014. These sampling points are indicates by one-shot pulses 1019. The sampled voltage values of half parabolic curve 1017 become a sampled compensation signal that can be applied to the SAW device to compensation for temperature variation.
Thus when the temperature is low and output voltage 1016 is high, voltage ramp 1015 will not rise above output voltage 1016 until sometime close to the end of the sampling period. By that time, half parabolic curve 1017 will be high and thus provide a high sampled compensation signal value to compensate for low temperature. Conversely when the temperature is high and output voltage 1016 is therefore low, voltage ramp 1015 will rise above output voltage 1016 sometime closer to the beginning of the sampling period when half parabolic curve 1017 is lower. This provides a lower sampled compensation signal value to compensate for higher temperature.
An illustration of this process is shown in FIG. 6, where the shape of parabolic curve 1017 is effectively mirrored by the negative slope of output voltage 1016 of temperature sensor 1014 to produce a sampled compensation signal curve 1021 consisting of discrete steps that together form a shape that is some-what parabolic, negatively sloped, and concave up.
Grieco provides the circuit shown in FIG. 7 for accomplishing this task. As explained above, a series of linear timing ramps 1015 is applied to an integrator 1017′ to produce half-parabolic curves 1017. The output from temperature sensor 1014 is compared with linear voltage ramps 1015 at comparator 1023, whose output is applied to a one-shot circuit 1019′ to produce one-shot pulses 1019. A sample-and-hold circuit 1025 samples the value of half-parabolic curves 1017 at points in time indicated by one-shot pulses 1019, and the sampled outputs 1021 are applied to a varactor 1029. As it would be understood, varactor 1029 is part of the SAW resonator based oscillator, and modulates the frequency of the oscillator in accordance with the sampled compensation signal 1021.
As explained above, however, this circuit compensates only the “cold end” of curve 1011. To compensate both the “cold end” and “hot end” of curve 1011, Grieco provides the circuit of FIG. 8, where first temperature sensor 1014 is used to compensate the “cold old” of curve 1011, and a second temperature sensor 1014′ is used to compensate the “hold end” of curve 1011.
To compensate the “hot end”, Grieco applies the output of second temperature sensor 1014′ to an inverting amplifier 1031 to produce an output voltage 1016′ whose general shape is the inverse of output voltage 1016, i.e. a positively sloped voltage as shown in FIG. 9.
With reference to FIG. 9, projecting the same stream of half-parabolic curves 1017 onto positively-sloped, output voltage 1016′ results in a second discrete sampled compensation signal 1021′ whose shape is some-what parabolic, positively sloped, and concave up curve.
Returning to FIG. 8, the output from inverting amplifier 1031 is compared with linear timing ramps 1015 by a second comparator 1037, whose output is applied to a second one-shot circuit 1039. The output of second one-shot circuit 1039 is applied to a second sampling trigger input of sample-and-hold circuit 1025.
However, since sample-and-hold circuit 1025 is now responsive to a first trigger input from first one-shot circuit 1019′ and response to a second trigger input from second one-shot circuit 1039, and since each of first one-shot circuit 1019 and second one-shot circuit 1039 individually output a separate pulse signal within each sampling period defined by linear timing ramps 1015, it follows that sample-and-hold circuit 1025 must sample half-parabolic curve 1017 twice during each sampling period. As a result, the compensation signal applied to varactor 1029 is changed twice during each sampling period irrespective of whether the temperature remained unchanged.
With reference to FIG. 10, eight exemplary sampling periods SP1–SP8 are shown. Because of the positive slop of linear timing ramps 1015, the second sample during each sampling period is always from the higher voltage of the two parabolic curves 1021 and 1021′ (shown in FIGS. 8 and 9). Nonetheless, it is clear that a first lower sample is momentarily taken prior to the second higher voltage sample. Thus, sample-and-hold circuit 1017 must be fast enough to make two separate sampling operations per sampling period. Since a sample-and-hold's response time is limited by the discharge rate of its charging capacitor, the operating frequency of this circuit is limited by the sample-and-hold's response time.
Additionally, the linear timing ramps 1015 cannot be made to have a saw-tooth waveform shape, but rather must have an initial zero-value, flat section at the beginning of each sampling cycle, as shown in FIGS. 5, 7 and 8 in order for the one-shot circuits 1019 and 1019′ to recharge. This further decreases the sampling rate.
What is needed is a simplified circuit that does not require additional A/D and D/A conversion, or additional decoding circuitry and memory. The simplified circuit should preferably also provide continuous temperature compensation (i.e. not discrete), and be flexible enough to support oddly shaped frequency-to-temperature characteristic curves of some oscillators without introducing instabilities, or placing harsh limits on its operating frequency.